This relates to group-wise detection of failure conditions, and in particular to detection of synchronization failures in a telecommunications network.
Telecommunication networks comprise switches that are interconnected by links, or trunks. In networks that employ digital switches that are interconnected via digital transport means, it is important for the switches to be synchronized with each other. This is achieved, typically, by using the same nominal frequency in the various switches and employing buffers at the interfaces between the switches and the transport means. The buffers easily compensate for phase differences and for small, temporary variations in frequency, but they cannot indefinitely compensate for variations in the average frequency. When the average frequencies are not the same, or when the frequency fluctuations are not short-term, the buffer cannot absorb the lack of frequency synchronization, and data is lost (or duplicated). The resulting degradation is known as xe2x80x9cslip.xe2x80x9d In voice communication, a slip has no noticeable impact, but in data communication a slip may result in various unwanted effects, such as partial obliteration of text (in a facsimile transmission), a request to retransmit data (in digital data transmission), a temporarily frozen picture (in video transmission) etc. With the advent of SONET transmission technology, the synchronization network had to be updated. SONET imposes two constraints on the synchronization network: SONET transmission equipment needs synchronization, and transporting synchronization signals over SONET can degrade their synchronization performance. The previous transmission systems did not require synchronization, and synchronization signals could be transported over these systems without significant degradation.
Before SONET, the ATandT synchronization network had a two-tier system where, in the top tier, 16 locations throughout the United States maintained a primary reference clock, while the second tier received the synchronization frequency via a T1 carrier system. The primary reference clocks were synchronized to the GPS. Having introduced SONET, the ATandT network now employs a single tier system with almost every switching and transmission offices (hundreds) maintaining a primary reference clock that is synchronized to the GPS.
The frequency derived from the GPS has short-term variability, and the GPS signal is sometimes unavailable due to various reasons. Therefore, Building Integrated Timing Supply (BITS) clocks are deployed at every office to filter out short-term variability of GPS-supplied frequency, as well as, to temporarily maintain a stable frequency source in the network during potential unavailability of GPS. High quality BITS clocks can maintain the synchronization frequency for long periods, typically weeks, without an impact on customer services. Eventually, if the GPS signal is not received or the BITS clock has a failure, the frequency will degrade and cause slips. Hence, monitoring equipment is used to compare the frequencies of different BITS clocks to each other. Each office, therefore, includes monitoring equipment to derive the BITS clock of other offices from signal arriving on the various links that are coupled to the remote office BITS clock.
At a minimum, the number of clocks that need to be compared is three, because when comparing one clock to another and a discrepancy is found, it is not known which is the correct clock. When three or more frequencies are compared and a frequency difference between them is found, majority voting can be employed to identify the errant clock.
Since each office receives a fairly large number of signals from different offices, checking each of the clocks for synchronization is time consuming and expensive. Furthermore, when high quality BITS clocks are deployed, monitoring individual signals from BITS clocks can be avoided without compromising the reliability of the synchronization network.
An improved approach is achieved by realizing that, even if there is a frequency synchronization problem, most of the clocks would be synchronized and the problem relates to one, or very few clocks. With this realization, group-wise testing of the clocks arriving at a switching office is undertaken by multiplexing the clocks onto a single line and developing a signal therefrom that is indicative of a problem, if it exists, in any of the component signals that were multiplexed. In one embodiment, the developed signal is a gated portion of the multiplexed signal. That signal is integrated over an integration frame and compared to the integrated signal of another integration frame. A difference between the two compared signals indicates that at least one of the clocks is out of frequency synch. Subsequent tests identify the offending clock, or clocks.